Titanium dioxide is a well known pigment and white opacifying agent. For example, titanium dioxide pigments are used in connection with coating formulations (including paint and ink formulations), paper compositions, polymer compositions and other products. Such pigments are generally produced in powder form with specific properties and characteristics depending on the final application. Titanium dioxide is a very effective, white opacifying pigment. It can be manufactured by either the sulfate process or the chloride process.
In the sulfate process for manufacturing titanium dioxide, a titanium slag ore is dissolved in sulfuric acid to form titanyl sulfate. The titanyl sulfate is then hydrolyzed to form hydrous titanium dioxide. The hydrated titanium dioxide is heated in a calciner to grow titanium dioxide crystals to pigmentary dimensions.
In the chloride process for manufacturing titanium dioxide, a dry titanium dioxide ore is fed into a chlorinator together with coke and chlorine to produce a gaseous titanium halide (such as titanium tetrachloride). The produced titanium halide is purified and oxidized in a specially designed reactor at a high temperature to produce titanium dioxide particles having a desired particle size. Aluminum chloride or some other co-oxidant is typically added to the titanium halide in the oxidation reactor to facilitate rutile formation and control particle size. The titanium dioxide and gaseous reaction products are then cooled and the titanium dioxide particles are recovered.
Whether produced by the sulfate process or the chloride process, the produced titanium dioxide particles are typically coated with one or more inorganic materials to modify or enhance the properties and characteristics of the pigment for particular applications. For example, the pigment particles are often coated with compounds that function to improve the opacity, light stability and durability of the pigment. Examples of inorganic materials used to coat titanium dioxide pigments include alumina and silica.
A primary property that a titanium dioxide pigment contributes to paint, paper, plastic and other products is hiding power. The hiding power of a titanium dioxide pigment is based on the ability of the pigment to scatter light in the base product (for example, a paint formulation) to which it is added. The ability of the pigment to scatter light in the base product to which it is added (the light scattering efficiency of the pigment) depends on various factors, including the particle size of the pigment, the difference in refractive index of the pigment particles and their surroundings (for example, a large difference in the refractive index of the pigment particles and the base product results in a high scattering efficiency), and the proximity of the pigment particles to one another. These factors have been addressed in various ways with varying degrees of success.
A potential problem that is associated with the use of titanium dioxide pigments in an aqueous based paint formulation is the tendency of the pigment particles to agglomerate in the paint formulations. Agglomeration of the pigment particles in a paint formulation can adversely impact desirable properties of the pigment including the opacity, brightness, tint strength and other optical properties of the pigment.
For example, problematic pigment agglomeration in aqueous based paint formulations often occurs after a paint film has been applied to a substrate and while the paint film dries. This phenomenon, sometimes referred to as optical crowding, can decrease the light scattering efficiency of the pigment particles. Consequently, the tint strength of the pigment can be diminished.
The problem of agglomeration of the pigment particles in an aqueous based paint formulation is exacerbated when the pigment is utilized in a paint formulation at a high pigment volume concentration (“PVC”). When the PVC in a paint formulation increases to a certain level, the light scattering efficiency of the pigment can substantially decrease. At high PVC values, the pigment particles are closer to one another, which results in an overlap of the respective light scattering cross-sections of the particles and thereby reduces the light scattering efficiency of the dispersed pigment. In addition to the light scattering efficiency of the pigment, the optical crowding effect can also decrease the light stability, brightness and opacity of the pigment.
Various techniques have been utilized in an attempt to diminish the optical crowding effect and address the other problems noted above. For example, fillers and extenders such as clay, calcium carbonate, alumina and silica have been added to paint base products to space adjacent pigment particles apart from one another. Hollow sphere, opaque polymers have been added to base paint products to create air voids in the base products that function to space the pigment particles apart. Also, pigment particles have been coated with certain inorganic compounds that function to modify the surface properties of the particles in a manner that discourages agglomeration of the particles.